![]() ![]() (The 85% coarse, 90% fine guideline, within its effective range, matches this in net effect).For metric V threads, major minus pitch yields a good tap drill diameter.The drill sizes that are near this are 15⁄ 64 in (85%), 6.0 mm (85.7%), and 6.1 mm (87.1%) any of these will work well.Example (metric, coarse): For M7.0×1.0 (an intended screw with a diameter of 7.0 mm and a pitch of 1 mm between each thread, which is considered coarse), 7.0 mm × 0.85 = 5.95 mm. ![]() Example (inch, fine): For 7⁄ 16-20 (same diameter as the previous example, but this time with 20 threads per inch, which is considered fine), 0.437 in × 0.90 = 0.393 in (i.e., if the threads are to be fine, then a slightly larger diameter drill bit should be used before tapping the hole for the screw).Therefore, a size 7⁄ 16 screw ( 7⁄ 16 ≈ 0.437) with 14 threads per inch (coarse) needs a tap drill with a diameter of about 0.371 inches. Example (inch, coarse): For size 7⁄ 16 (this is the diameter of the intended screw in fraction form)-14 (this is the number of threads per inch 14 is considered coarse), 0.437 in × 0.85 = 0.371 in.A good tap drill is 85% (± 2 pp) of major diameter for coarse threads, andĪ good tap drill is 90% (± 2 pp) of major diameter for fine threads.Some sizes outside that range have different ratios.īelow, these guidelines are explored with examples. The 85% and 90% rules works best in the range of 1⁄ 4–1 in (6.4–25.4 mm), the sizes most important on many shop floors.For both of these rules of thumb (85%/90% and major minus pitch), the tap drill size yielded is not necessarily the only possible one, but it is a good one for general use.Your result will only land near a tap drill size (not directly on one). ![]() ![]() For example, a screw with a pitch of 1/20 in (20 threads per inch) has a pitch of 0.050 in and a 1⁄ 13 in pitch (13 threads per inch) has a pitch of 0.077 in. The major minus pitch technique also works for inch-based threads, but you must first calculate the pitch by converting the fraction of threads-per-inch (TPI) into a decimal.
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